Lubricating oils for automatic transmissions, called automatic transmission fluids, have been used conventionally to assist smooth operation of automatic transmissions which are installed in automobiles and include a torque converter, a gear mechanism, a wet clutch, and a hydraulic mechanism.
It is well known that lubricant additives give effects on the friction properties of wet clutch and steel plates. Additive effects are caused by both their physical and chemical absorption on clutch materials, ex. cellulose, aramid (a natural and synthesized) fibers, silica and steel plate surface. There has been an Industry drive to change from cellulose rich to aramid rich wet clutch papers for use in in automotive automatic transmissions. The ratio of cellulose and aramid is important for thermal and oxidation stability performance of wet clutches. High aramid wet clutch paper shows excellent durability performance. However, the cost of aramid fiber is high.
Further, regulatory changes have resulted in modem vehicles being required to have improved fuel economy and reduced CO2 emissions to prevent global warming. In addition to improvements in the design of engine and transmission systems, lubricant performance has also been required to address this issue. In the case of automotive automatic transmissions, power loss caused by the torque converter in a starting time need to be minimized, and lock up clutch systems have been introduced to improve fuel efficiency. Lock up torque converters are installed in lock-up wet paper clutches in the torque converter systems. These can reduce power loss and provide excellent fuel economy, because they can engage the wet clutches after fluid coupling at low speeds and a shorter time.
On the lubricant side, having the right lubricant for an automatic transmission with lock up paper wet clutch in the transmission is also very important. If a lubricant gives poor torque capacities and anti-shudder friction performance, power loss or uncomfortable vibration with high noise from lock-up of the wet clutch in the transmission would occur. Thus, lubricants for an automatic transmission with lock up paper wet clutch systems should provide both good fuel economy and smooth driving and operating condition.
The inventors have discovered a lubricating oil composition which has excellent wet paper clutch friction characteristics, such as anti-shudder performance, and which can also maintain excellent wet clutch torque capacity and durability of wet clutch friction characteristics.